Acoustic resonators are today very widespread in applications for the general public but also in professional applications because of their efficiency, in particular for the realization of very powerful band-pass filters, and specifically in mobile telephony for the selection of transmission channels.
A classical manner to carry out a band pass filter comprises utilizing two acoustic resonators, having slightly shifted characteristics, and assembling them in serial/parallel for the purpose of constituting an elementary filtering cell. FIG. 2a illustrates such a known circuit where it can be seen a first resonator 10 (fs1) mounted in serial and a second resonator 20 (fs2) in parallel within the same quadripole. FIG. 2b illustrates the curves characteristic of each of these resonators and the curve of the resulting filter band pass.
This permits a particularly narrow band pass filter to be achieved, which can thus be combined with other elementary cells for the purpose of assembling a more complex filter.
Researches have been made on two types of acoustic resonators which present remarkable performances, namely the resonators of type SAW (Surface Acoustic Wave Resonator) and, more recently, those of type BAW (Bulk Acoustic Wave Resonator). In the first type, the acoustic resonator is positioned on the surface of a semiconductor product while, in the second type, it is positioned inside a volume delimited between a lower electrode and a higher electrode so that the acoustic wave develops in that same volume. BAW type resonators are the subject of intensive investigation and research by the manufacturers of semi conductor products because these components allow to consider a very thorough integration of filtering circuits and, by consequence, let foresee a significant reduction of the manufacturing costs for these circuits.
However, it remains that the realization of an acoustic filter made of two resonators having characteristics being slightly shifted shows to be particularly delicate because of the constraints weighing on the precision of the manufacturing process. Indeed, it is necessary to be able to realize, for each filtering cell, not only one resonator but a set of two resonators of which each one presents precise characteristics in order to guarantee a suitable frequency shift allowing to obtain the desired characteristics curve. The high level of precision which is required calls for critical manufacturing steps within the manufacturing process of the band pass filter. More particularly, it is necessary to consider the manufacturing of two identical resonators, then a stage of “overloading” of one of the two resonators.
Practically, the addition of a layer above the higher electrode permits the frequencies of resonance to be shifted. Then a posterior treatment of type “trimming” by laser ablation is executed, which intervenes after the passivation of that circuit. This process entails a disadvantage which is the fact that it does not allow to correct individually each of the resonators because the laser beam is too much broad. We can thus only correct in a more or less uniform way the set of resonators of the filter without knowing the effect produced on each resonator individually; loaded or not.
Such operations are complex, delicate and expensive to implement.
In order to avoid the constraint on the manufacturing process, one may contemplate the use BAW or similar adjustable type resonator, for the purpose of adjusting the characteristics of the component. French Patent Application No. 03 15480 entitled “INTEGRABLE ACOUSTIC RESONATOR, AND PROCESS OF INTEGRATION OF SUCH RESONATOR,” filed on Dec. 29, 2003 (corresponding to U.S. patent application Ser. No. 11/025,599, filed on Dec. 29, 2004 (agent's reference 852263.415) describes an Adjustable Resonator Component comprising a resonator type BAW to which an inductance is associated, judiciously selected in the vicinity of one of the frequencies of resonance, and an adjustable capacity allowing to shift and to correct the global frequency of the resonator.
This already permits a significant achievement in the overall process of integration of a BAW type resonator in a semi-conductor product. However, it shows to be not so easily exploitable in a simple and low-cost process. It should be particularly noticed the significant space occupied by the inductances required for the implementation of that technique.
In other known circuits, the difficulty is overcome by simply suppressing one among the two resonators of the filtering quadripole and by substituting a capacitor as shown in FIG. 3a illustrating a known circuit, comprising a resonator 30 (fs1) mounted in the serial branch of the quadripole, and associated to a capacitor 40 located in the parallel branch. In certain cases, as it can be seen in the figure, in order to improve rejection, it can be useful to use another capacitor 50 connected in parallel on the terminals of the resonator. In both cases, one can simplify the manufacturing process because one can avoid the need to manufacture a filtering cell made up of two resonators having precise and slightly shifted characteristics. On the other hand, as it can be seen in FIG. 3b, the response curve of such a quadripole has not the exact profile which is looked for and this is a major drawback.
FIG. 4a illustrates the dual embodiment based on a capacitor 60 on the serial branch, followed by a resonator 70 on the parallel branch, sometimes associated with a capacitor 80 being in parallel with the resonator. It results another filtering cell from it of which the curve is represented in FIG. 4b. 
The circuits which were mentioned above, if they avoid the need of integrating two resonators having slightly shifted characteristics, do not permit a satisfactory filtering circuit to be obtained. Should the designer really wish to have the two “zero” which are characteristics of the transfer function of the filter, then it will be necessary to use the “classical” cell of FIG. 2a and to cope with the disadvantages of this cell which were mentioned above.